You can make Avaz the only app on the device and prevent the user from exiting Avaz once the app is opened. You can lock the app so that he remains locked into the Avaz app and can't return to the Home screen and delete the app or have access to other games or internet or other apps etc. This is a useful way to prevent the child from deleting the app.
iPad
The iPad Setting called Guided Access will enable you to control the exit.
Click here to see What is Guided Access and how to use it ?
Android devices
For Android it is done through Screen pinning.
Here is a link to one of the solutions:
Sometimes it is device specific solutions. Take a look at some of these links;